Physiology of heat stress in cattle and sheep

Summary

This report details experiments conducted to define the physiology of heat stress in cattle, which has a particular emphasis on the live export trade, and the efficacy of electrolyte supplementation. The first two experiments were conducted on Bos taurus cattle, and the third on Bos indicus cattle. These experiments used climate controlled rooms modified to provide conditions of high heat and humidity, similar to the conditions experienced by animals transported by ship on long haul voyages to the Middle East. A fourth experiment was conducted on Bos taurus cattle, to differentiate feed and heat effects.

The animals subjected to the conditions mimicking shipboard temperature and humidity responded generally as expected from the existing literature on heat stress in cattle. They used sweating and panting to increase heat loss from their bodies, but the accumulated effects of continual high heat and humidity resulted in a rise in core body temperatures. The major findings were:

- Bos taurus cattle tolerated the accumulated heat load less well than the Bos indicus cattle.

- Feed intake by the Bos taurus cattle was drastically reduced during the periods of heat exposure; feed intake of the Bos indicus cattle was much less affected.

- There was no indication of dehydration in these heat exposed animals with free access to water, and they all increased their water intake.

- Both types of cattle eventually resorted to panting to relieve heat load, but this response was delayed in the Bos indicus cattle.

- Panting reduced blood carbon dioxide, but the animals compensated for the tendency to respiratory alkalosis, losing blood bicarbonate to maintain blood pH through the heat insult.

- Cessation of the heat resulted in a rapid restoration of blood carbon dioxide, without an associated rise in blood bicarbonate, leading to a lower blood and urine pH.

- Blood bicarbonate was slow to return to pre-heat values.

- Plasma sodium and calcium decreased by the end of the hottest period, while plasma concentrations of other electrolytes were not consistently altered. - Sweating rate was not as high as reported for summer adapted cattle.

- Bos taurus cattle in April sweated at a higher rate than Bos taurus cattle in June; Bos indicus cattle tested in August sweated at a rate intermediate between the two groups of Bos taurus cattle.

- Feed restriction without heat exposure was associated with a reduction in plasma calcium and magnesium, but there was no change in plasma sodium or blood bicarbonate.

- Thyroid hormones and leptin were affected by exposure to the heat, and these changes were different to those measured in animals feed restricted without heat exposure.